Asthma is a familial disease with rising prevalence in recent years. The inheritance of asthma is not well understood. To investigate whether asthma may be inherited through a major gene with 2 alleles, segregation analyses were conducted in 3369 individuals from 906 nuclear families enrolled in a longitudinal study of respiratory health. Physician diagnosed asthma, and its age of onset, were ascertained for each family member when children were at a mean age of 7 years. Segregation analyses were conducted with and without the covariate 1gE. The hypothesis of a single allele locus for asthma was rejected. Linkage studieshave been started to investigate different areas in the genome potentially linked to various related phenotypes.